This invention relates to a process for cleaning coal. More particularly, the present invention concerns a process for cleaning coal containing shale or clay slimes (such coal being referred to hereinafter as undeslimed coal), wherein the undeslimed coal is mixed with a magnetic heavy media suspension such as magnetite and the three phase slurry of coal, magnetite and water is fed to a cyclone to separate the coal from nonbituminous material, such as ash.
Examples of prior art processes that utilize heavy media separation aids are shown and described in U.S. Pat. Nos. 3,282,417, 3,247,961 and 3,246,750, all to Chase et al. The 3,282,417 patent discloses a method of controlling the cleanness of the heavy media suspension that is recirculated in the separation process. The method includes the steps of measuring both the specific gravity of the suspension and the content of magnetic particles therein at a location representative of the vessel, developing a signal proportional to the difference between the specific gravity and the content of magnetic particles, which signal is representative of the content of contaminants, introducing water to said vessel in volumes which vary with said signal to maintain a relatively constant set content of contaminants in the suspension within said vessel, and adjusting the content of magnetic particles in the suspension entering said vessel after the water volume changes, thereby compensating for changes in the specific gravity of the suspension brought about by variations in the water volume, and maintaining the specific gravity of the suspension within said vessel at a relatively constant set value.
Significant problems can occur in this prior art process due to the inherent inaccuracies, noise and drift characteristics, and varying gains or sensitivities of the magnetic sensor and the density sensor. When considerable slimes are present in the mineral mixture to be separated, the relatively simple procedure of subtracting the values of the signals from the two sensors can produce such erroneous readings that the solid-solid separator used in the process may operate with relatively erratic efficiencies. It is important to note that efficiency of a cyclone type of solid-solid separator is particularly sensitive to small changes in viscosity of the three-phase slurry of coal, magnetite and water that can be brought about by shale or clay slimes. Thus, it is important that any discrepancies inherent in the specific gravity and the magnetite level instrumentation, such as noise drift, any differences in gain or sensitivities of the two sensors and any difference in relative accuracies of the two sensors, be reduced.
It is also important in a cyclone type of heavy media separation process to sense the heavy media suspension level and provide additional media (magnetite) to the feed stream to the cyclone quickly upon indication that the media level is different from a set value.